Examples of systems which conduct masses to the intake manifold are the tank venting and the exhaust-gas recirculation. Known methods for diagnosing these systems are based on a detection of the mass flows to the intake manifold. U.S. Pat. No. 4,794,790 describe, for example, a diagnostic method for a tank-venting valve wherein an idle actuator is closed simultaneously with the opening of the tank-venting valve. In a good condition, the additional mass flow from the tank-venting system is intended to compensate for the reduction of the mass flow via the idle actuator.
For diagnosing the exhaust-gas recirculation, it is further known to measure the temperature increase in the intake manifold via the exhaust-gas mass flow to the intake manifold for an active exhaust-gas recirculation.